The present invention relates to a light-emitting diode, a light-emitting diode array, and a method of their fabrication.
A conventional light-emitting diode is disclosed in the publication, "Design of an Optical Printer", Triceps WS 6, 1985, pp. 121-126. The structure of the conventional LED and the method of its fabrication will first be described with reference to FIG. 7.
The illustrated conventional LED 50 is formed of an n-type GaAs substrate 51, and an n-type GaAsP layer 52 formed on the GaAs substrate, and a diffusion mask 54 having an aperture formed on the GaAsP layer 52. The LED 50 is provided with a p-type diffusion region 56 on the surface of the n-type GaAsP layer 52.
A p-side electrode 58 is provided to extend on the surface of the diffusion mask 54 and the p-type diffusion region 56. An n-side electrode 60 is provided on the lower surface having the substrate 51.
A vapor-phase diffusion is generally used to form the diffusion layer 56 in the n-type GaAsP layer 52.
With the conventional LED 50, the GaAs substrate 51 is used as the substrate 51, and a relatively thick n-type GaAsP layer 52 is formed on the substrate 51. For this reason, the diffusion region 56 can be formed by vapor-phase diffusion, deeply in the n-type GaAsP layer 52.
The conventional LED has the following problems with regard to the substrate.
Because a GaAs substrate is used as the substrate 51, it is easily broken during dicing, and is easily chipped off during dicing, so dicing has to be conducted slowly.
Moreover, the dimension of the wafer is utmost 3 inches (about 7.62 cm) at present, and a wafer of a diameter larger than 3 inches (e.g., of 8 inches (about 20.32 cm)) is not available. In addition, the cost of the substrate is higher than a silicon substrate or the like.
To solve the problems described above, an attempt has been made to form a compound semiconductor layer on a silicon (Si) substrate. However, this brings about the following problems.
If a compound semiconductor layer is formed to a large thickness on a silicon substrate, a crack may be produced in the compound semiconductor layer because of the difference in the thermal expansion coefficient between the materials of the substrate and the compound semiconductor layer, and no useful device is obtained. The present inventors have confirmed that cracks are generally produced in the compound semiconductor layer if the thickness of the compound semiconductor layer 3.5 .mu.m or more. For this reason, there has been a restriction that the compound semiconductor layer on the Si substrate cannot be made thick.
If the compound semiconductor layer on the substrate is made thin, and if a diffusion layer is formed by vapor-phase diffusion, the depth of the diffusion region may exceed the thickness of the compound semiconductor layer, and no pn junction can be formed. For these reasons, it was very difficult to form an LED or LED array using the Si substrate of a large diameter.